Blocking spam email using the MAPS Realtime Blackhole List

The check_relay ruleset in the sendmail.cf configuration file for
UnixWare 7
sendmail(1M)
can be configured to reject mail based upon the
MAPS (Mail Abuse Prevention System) Realtime Blackhole List
maintained at maps.vix.com. The MAPS RBL is a
list of sites which are known to either originate or relay spam.
During an SMTP session, check_relay makes a DNS
query on the IP address of the connecting host to check for
the existence of an address resource record (``A'') of that host
under the rbl.maps.vix.com node. If such a record exists, then
the check_relay ruleset issues an error and drops the SMTP
connection.

The check_relay ruleset follows the following algorithm:

If the name of the host making the SMTP connection is the local
host, the local domain, or a sub-domain of the local domain, then
the mail is accepted.

If the IP address of the host making the SMTP connection is not
listed at the MAPS RBL, then the mail is accepted.

Otherwise, the mail is rejected.

If the mail is rejected, sendmail issues the following error message
during the SMTP dialogue, and writes it to the system log file as well:

Mail from <connecting host> refused by blackhole site rbl.maps.vix.com

By default, this feature is disabled in sendmail.cf.
To enable check_relay, set the Check_Relay
macro to a non-null value by changing the following line
in sendmail.cf:

D{Check_Relay}

to the following:

D{Check_Relay} ON

Next, stop and restart the sendmail daemon to load the configuration
changes by executing the following commands:

/etc/mail/sendmailrc stop
/etc/mail/sendmailrc start

NOTE:
Use of the check_relay ruleset requires that DNS be properly
configured, and is incompatible with the DeliveryMode=deferredsendmail option, as it requires a DNS lookup be done
immediately upon mail receipt. Administrators
should also consider whether their server can handle the load of an additional
DNS lookup for each message received before enabling this feature.